1. Field of the Invention
The invention relates to an optical amplifier for pulsed laser with short (shorter than 10 nanoseconds) or ultra-short (shorter than 1 picosecond) and energetic (energy higher than 10 microjoules per pulse) pulses.
2. Description of the Related Art
A pulsed laser generally associates an oscillator and an amplifier system to generate both short and energetic pulses. The oscillator generates pulses from one femtosecond to several tens of nanosecond in duration and with a good spatial quality, but the energy of which is generally limited (from a few nanojoules to a few millijoules). To increase the energy delivered by an oscillator, it is necessary to use an amplifier system capable of both greatly increasing the output power and extracting the energy with the best efficiency possible. These two functions required for a good amplifier are, in most cases, antagonistic because it is difficult to obtain both a very high gain and an excellent energy extraction due to phenomena of gain saturation with the incident power.
The key parameter of an amplifier system is the amplification factor, also called “gain”. The gain is given by the following formula:
      G    eff    =      ⅇ                  ∫        0                  l          c                    ⁢                        g          ⁡                      (            z            )                          ·                                  ⁢                  ⅆ          z                    
where Ic is the total length of the gain medium and g is the lineal gain in m−1, at the laser wavelength. The lineal gain gl depends on the “small signal” lineal gain g0 and on the ratio of the laser intensity to be amplified Isignal to the saturation intensity Isat, according to the following formula:
      g    I    ∝            g      0              1      +                        I          signal                          I          sat                    
The dependence of the gain on the incident laser intensity is thus a fundamental piece of information for the making and the use of an amplifier system, because it determines the potential of an amplifier system for being used as a pre-amplifier or an end-of-chain power amplifier. The average power obtained at the output of the amplifier, which is a limitative parameter in some systems, and the efficiency of an amplifier, will also be considered.
The efficiency is the ratio between the output power and the pump power.
A first solution to optimize the gain and the amplification efficiency consists in implementing several amplifiers in series so as to associate media of different characteristics: a first system called a “pre-amplifier”, with a high “small signal” gain, is followed by one or several systems called “power amplifiers”, permitting to obtain a high efficiency but having a limited gain. Some systems comprise four amplifiers in series, which is both complex and costly. In order to minimize the number of amplifier components, a compromise has to be made between the two regimes of operation. Numerous high-power amplifier systems are based on multiple passages of the beam in the amplifying medium. These systems are also complex.
Another solution consists in multiplying the passages of the beam in an amplifying medium so as to maximally extract the energy stored. A high gain is necessary in “small signal” regime. In this regime, the quantity of light injected into the amplifier is so small that the amplification factor depends only on the quantity of energy stored. The latter is generally far higher than the incident energy and the energy obtained at the output, and thus the efficiency is low. It is the case, in particular, in the regenerative amplifiers that are active systems in which it is necessary to accurately control the synchronization of the injection and the ejection of the pulse in the so-called “slave” cavity. K. Sueda et al. (“LD pumped Yb:YAG regenerative amplifier for high average power short-pulse generation”, Laser Physics Letters 5, 271-275, 2008) have obtained gains higher than 100 in Nd:YVO4 or Yb:YAG crystals, for a pump power higher than 100 W. This solution is certainly high performing, but it is very costly and its implementation is complex. It is also the case for systems that use a set of mirrors to refocus the beam in the gain medium. The signal is thus amplified thanks to multiple passages in the same amplifying area of the medium. However, these multiple-passage systems are very complex.
Various media permit to amplify pulses through one or a few passages (less than 10). Systems based on solids doped with rare earth or metal ions (such as Nd:YVO4), systems based on optically active liquids (as in dye laser), systems based on optically active gaseous media (as the Rubidium), or systems based of doped glass fibres in which the signal to be amplified is guided within the medium to favour the amplification, are known. In particular, to amplify pulses whose wavelength is around 1 μm, the doping with rare earth ions of crystalline or amorphous media, and notably neodymium and ytterbium ions, allows high powers to be generated.
FIG. 1 indicates the performances in terms of gain and average output power for various types of amplifying media associated with various optical pumping systems according to the state of the art. This graph provides a cartography of the different systems. The systems based on crystals are represented by squares and the optical fibre systems by diamonds.
According to the type of amplifying medium, various configurations of optical pumping have been developed.
The geometry of the systems based on bulk crystals strongly depends on the pumping configuration, which is generally transverse or longitudinal.
In case of transverse pumping, an efficient configuration is the “slab” geometry, in which the beam performs round trips inside the gain medium, after reflection on mirrors, so as to extract the maximum of energy and to favour the overlap of the laser mode and the pumped volume. A gain of the order of 2 with an output power of 10 W after two passages has thus been obtained in a Nd:YVO4 crystal. This geometry is favourable to the use of high-power laser diode bars (stack) that can be placed so as to directly pump the medium in the transverse direction. Despite the simplicity of the pumping system, this configuration intrinsically generates a strong dissymmetry and significant thermal lens effects in the medium. In order to limit the beam ellipticity, it is possible to fold the beam so as to multiply the round trips within the crystal. In this case, it is difficult to correctly extract all the gain present in the crystal. The gain obtained is of the order of 2, with an output power of 102 W for a pump power of 100 W. At such pump power, the thermal effects are then very significant and restrict the power increase of this type of system.
In order to reduce the strong thermal lens in this direction, another solution is to force the signal to propagate in this same direction (the smallest direction of the crystal) by “zigzagging” after several internal total reflections on the faces of the medium. A gain of 12.5 and an output power of 2.5 W have thus been obtained in the Nd:YVO4 crystal, for a pump power of 40 W. Shiradan et al. (Applied Optics 46, 7552-7565, 2007) have used such a geometry in a Yb:YAG crystal and have obtained a gain of 20 and a power of 2 W has been demonstrated for a pump of 16 W. Furthermore, the latter configuration has the advantage that it simplifies the system with respect to the previous ones and that it makes it possible to obtain high gains from bulk crystals, but with a relatively limited extraction. Besides, the making of systems in which the signal propagates according to a grazing incidence (“bounce geometry”) also makes it possible to limit the thermal effects because the beam reflects on the cooled face. A gain of about 5.8 as well as an output power of 60 W have thus been obtained with a single passage in a Nd:YVO4 crystal. Once again, the advantage of this system is the high output power, but the thermal effects remain a limitative factor for a use toward higher powers.
A longitudinal pumping within a bulk crystal allows an optimal overlap between the pumped area and the area of propagation of the incident beam to obtain the best efficiency, and thus a maximum output power while preserving a significant gain. Power laser diodes whose beam is conformed are excellent pumping sources and allows high brightness to be reached. The power densities in the gain medium may then be very high. Therefore, a careful dimensioning of the pumping optics has permitted to obtain, through a single passage in a Nd:YVO4 crystal pumped at 888 nm, a gain of 2 and an output power of more than 110 W. Moreover, the pumping at 888 nm permits to reduce the thermal effects and to increase the extraction.
FIG. 1 indicates that the high-power amplifiers based on doped crystals permit to reach high extraction efficiencies, but with generally limited gains, lower than 13 and even 2 in the case of an output power of the order of 100 W. The materials used are generally Yb:YAG crystals or neodymium-doped crystals, which are characterised by a great efficient emission cross-section at the laser wavelength, but which are limited in most cases by the thermal phenomena reducing the prospects for power increase. The thermal stabilisation of the gain medium requires the use of crystals of sufficient size to support high powers of pump.
Other optical amplifier systems do not use a bulk crystal but use an amplifying optical fibre. Various types of amplifying optical fibres have been developed, in particular dual-core amorphous doped glass fibre, wide-core fibres and photonic-crystal fibres.
The structure of the dual-core amorphous fibres doped with rare earth ions makes it possible to overcome the thermal problems. In such fibres, the pump wave is guided so as to distribute the absorption over a great volume and thus to restrict the temperature rise. The signal beam is also guided in a multimodal lightly-doped core, which permits to preserve an excellent quality of beam as well as an excellent efficiency, thanks to the perfect overlap between the beams of the signal and of the pump. Thus, the pulse amplification with high gains and through a relatively simple scheme has been demonstrated from optical fibres doped with neodymium or ytterbium ions. Gains of 10 to 20 have been obtained in fibres with a core of less than 11 μm in diameter and of several meters long. However, such amplification is limited by a high rate of amplified spontaneous emission. Besides, the main factors limiting this architecture arise as soon as it is desired to amplify both short (of the order of about ten nanoseconds and less) and energetic (as soon as ten microjoules) pluses. Indeed, the peak power that propagates in the amplifying medium is sufficient to generate strong undesirable non-linear effects such as the Raman effect, the stimulated Brillouin emission, or the self-phase modulation. These effects are the main limitation for the amorphous fibres to reach high average output powers.
In order to limit the undesirable non-linear effects, large core (more than 15 μm in radius) or small-length fibres has been developed. From a fibre of 50 μm in diameter and an injected energy of 4 μJ (a pulse of the order of one nanosecond in duration), 750 μJ at 1 kHz (i.e. 750 mW) have been obtained for a pump of 7 W, i.e. a gain of 190. Cheng et al. (Optics Letters, 30, 358-360, 2005) have used a fibre with a core diameter of 200 μm, having a strongly multimodal emission, to amplify pre-amplified pulses up to 2.7 mJ with a gain of 10. A phosphate fibre with a core of 10 μm in diameter, strongly doped with Yb3+ ions, and with a length limited to only 47 cm, has permitted the making of a relatively simple amplifier with a gain of 28 dB (i.e. a neat gain of 690) and a corresponding output power of 10 W. Such performance corresponds to the highest gain obtained shown in FIG. 1.
The photonic-crystal fibres (“Rod-type”), whose doped-core size varies between 50 and 100 μm in diameter, are also a promising solution. Pulses of 85 picoseconds and 40 mW have been amplified up to 1.5 W in a first fibre-amplifier, then up to 27 W in a photonic-crystal fibre with a core diameter of 70 μm, for a pump of 100 W, with a gain in the very wide core fibre of about 18. These values are very significant and constitute an intermediate solution for obtaining both high gain and high average power. However, this solution requires the use of a relatively complex medium and remains limited to the amplification of pulses whose energy is lower than a few millijoules at most, as a function of the doped-core diameter used, so as to remain under energy density thresholds for the doped glass (about 22 J/cm2 for pulses of 1 ns at the wavelength of 1 μm).
To sum up, the doped glass fibres, whether they are conventional or based on photonic crystal, permit to strongly concentrate the pump and the signal, but also to support high pump powers, while preserving an excellent spatial beam quality. FIG. 1 indicates clearly that the fibre amplifiers permit to obtain the highest gain values. However, their dimensions are, for the moment, limited to cores of a few tens of micrometers. This is a limitative factor for the amplification of high energy (thus of high average powers) pulses, due to the thresholds of occurrence of the non-linear effects and of damage of the glass. This limitation appears in FIG. 1: the average powers of the fibre amplifiers remain lower than 30 W.
The performances obtained in terms of gain and average output power for various types of amplifying media coupled to various pumping systems of the state of the art are illustrated on the graph of FIG. 1. This graph provides a cartography of the different systems. Two general trends can be deduced therefrom. On the one hand, the bulk crystals make it possible to reach high average powers, but with a relatively low gain. On the other hand, the fibres make it possible to reach high gains, but with a limited output power. To sum up, the amplifiers of the state of the art do not permit to obtain high gain and high average power at the same time.